Sharepoint: What happens to lists based on content type when content type is updated?

I have kind of hypothetical question (at least for now :))

Let's say I create list based on some custom content-type. I add some 1000 items into that list (in production). Then customer comes and he say that he need to modify that custom content type.

What happens to list if I modify custom content type? Will it be updated automatically (I doubt) ? And what about already created list items?

Do any of you have some experience with this?